Importance of Trends in Human Disease and Condition

Numerous human diseases and conditions have been linked with exposures to environmental contaminants, some more strongly than others. Identifying diseases that might be associated with environmental contaminants, and determining the existing data sources available for them, is a key part of the effort to better characterize links between environmental exposures and adverse health outcomes. Learn more »

Tracking overall rates of disease in the United States, independent of exposure, enables the evaluation of disease patterns and emerging trends. It may identify diseases, conditions, and possible risk factors that warrant further study or intervention and can help identify where policies or interventions have been successful. Because the United States has a diverse population, an important component of such an analysis is identifying disparities among people of differing races and ethnicities, genders, education and income levels, and geographic locations.« Collapse text

Measures of Human Disease and Condition

ROE health indicators use disease morbidity (incidence or prevalence) and/or mortality (resulting death) statistics. Both measures provide useful insights about trends in disease; however, both are influenced by a number of factors, including the accuracy of reporting mechanisms and issues related to access to, quality of, and advances in medical care. Learn more »

Both morbidity and mortality can be measured using occurrences or rates:

Occurrences represent frequency counts.

Rates enable a comparison across populations. Rates are ratios that calculate the frequency of cases (of disease, condition, outcome) divided by the size of the defined population for a specified time period. Usually some constant (generally a multiplier of the power 10) is applied to convert the rate to a whole number.

Morbidity data are often used to describe the incidence and prevalence of a disease or condition. Both incidence and prevalence are often expressed as a rate per 1,000 persons over a particular time period:

Incidence refers to the number of new cases of a disease or condition in a population during a specified time period.

Prevalence refers to the total number of people with a given disease or condition in a population at a specified point in time.

Mortality is generally expressed as a rate and is defined as the proportion of the population who die of a disease or condition during a specified time period. The rate is usually calculated for a calendar year and is often expressed per 100,000 persons.

Incidence, prevalence, and mortality statistics can be used to compare the rates of disease at two or more points in time, across different populations (ages, gender, racial/ethnic groups), or between different geographic areas.

In general, disease incidence, prevalence, and mortality increase with age. For this reason, when comparing different populations, the data must be adjusted to account for the age differences between the populations. The adjusted data, called “age-adjusted rates,” are used where possible in addressing the ROE human disease and conditions question. Age-adjusted rates are weighted sums of age-specific rates and calculated using standard population factors. (In the ROE, the 2000 U.S. standard population was used.) Unadjusted rates are referred to as “crude” rates.« Collapse text

The term “cancer” refers to diseases in which abnormal cells divide without control, losing their ability to regulate their own growth, control cell division, and communicate with other cells. Cancer is the second leading cause of death in the United States (General Mortality indicator). More than one in three people will develop cancer and nearly one in four will die of it.1 In response, scientists continue to explore the role that the exposure to environmental contaminants may play, along with other possible risk factors, in the initiation and development of cancer. Some environmental contaminant exposures are known risk factors for certain types of cancers. Examples include radon and lung cancer and arsenic and skin cancer. Though many types of cancer are suspected of being related to ambient environmental exposures, associations are not always clear because the etiology of cancer is complex and influenced by a wide range of factors. Many factors can increase individual cancer risk, such as age, genetics, existence of infectious diseases, and socioeconomic factors that can affect exposure and susceptibility.

Childhood cancers are dissimilar from cancers in adults and are therefore tracked separately. They affect different anatomic sites and may be of embryonic origin. Though overall cancer incidence rates are lower in children than in adults, childhood cancers are the leading cause of disease-related death in children age 1 to 19 years.2,3 Children may be particularly susceptible to exposures in utero or during early childhood because their systems are rapidly developing and affected by evolving hormonal systems.4 As with many adult cancers, the causes of childhood cancers are unknown for the most part; environmental influences may be a factor and have been the subject of extensive research. Environmental exposures are difficult to evaluate because cancer is rare in children and because of challenges in identifying past exposure levels, particularly during potentially important time periods such as in utero or maternal exposures prior to conception.5

More than one-third of the U.S. adult population lives with a cardiovascular disease, with more than 6 million hospitalizations each year.6 Coronary heart disease and stroke, two of the major types of cardiovascular disease, rank as the first and fourth leading causes of death, respectively (General Mortality indicator), and are leading causes of premature and permanent disabilities. Known risk factors include smoking, high blood pressure, high blood cholesterol, diabetes, physical inactivity, and poor nutrition. Outdoor air pollution and environmental tobacco smoke are also known risk factors for cardiovascular disease. Particulate matter, for example, has been demonstrated to be a likely causal factor in both cardiovascular disease morbidity and mortality. Collective evidence from recent studies suggests excess risk associated with short-term exposures to particulate matter and hospital admissions or emergency department visits for cardiovascular effects.7,8,9 Environmental tobacco smoke has been shown to be a risk factor for coronary heart disease morbidity and mortality and may contribute to stroke.10,11

Chronic obstructive pulmonary disease (COPD) and asthma are two prevalent chronic respiratory diseases. Chronic lower respiratory diseases represent the third leading cause of death in the United States (General Mortality indicator). Epidemiological and clinical studies have shown that ambient and indoor air pollution are risk factors in several respiratory health outcomes, including reported symptoms (nose and throat irritation), acute onset or exacerbation of existing disease (e.g., asthma), and deaths.12,13,14 The relationship between environmental tobacco smoke and diseases of the respiratory tract has been studied extensively in humans and in animals; environmental tobacco smoke has been shown to produce a variety of upper and lower respiratory tract disorders.15,16

COPD is a group of diseases characterized by airflow obstruction, resulting in breathing-related symptoms; it encompasses chronic obstructive bronchitis and emphysema.17 COPD is the third leading cause of death in the United States and the leading cause of hospitalization in U.S. adults, particularly older adults. It represents a major cause of morbidity, mortality, and disability.18 Air pollution may be an important contributor to COPD, though approximately 80 percent of COPD deaths is generally attributed to smoking.19

Asthma continues to receive attention in both children and adults. Asthma prevalence has grown over the past few decades. Asthma prevalence increased nearly 74 percent during 1980–1996, with more than 20 million people in the United States reporting asthma each year over the last decade.20,21 Environmental exposures such as outdoor air pollution (e.g., particulate matter, ozone), environmental tobacco smoke, dust mites, pets, mold, and other allergens are considered important triggers for asthma.22,23,24

Infectious diseases are acute illnesses caused by bacteria, protozoa, fungi, and viruses. Food and water contaminated with pathogenic microorganisms are the major environmental sources of gastrointestinal illness. Though well-established systems for reporting food- and waterborne cases exist, data reported through these largely voluntary programs must be interpreted with caution, because many factors can influence whether an infectious disease is recognized, investigated, and reported. Changes in the number of cases reported could reflect actual changes or simply changes in surveillance and reporting. In addition, many milder cases of gastrointestinal illnesses go unreported or are not diagnosed, making it difficult to estimate the number of people affected every year.

The discovery of bacterial contamination of drinking water as the cause of many cases of gastrointestinal illness represents one of the great public health success stories of the 20th century.

Waterborne diseases such as typhoid fever and cholera were major health threats across the United States at the beginning of the 20th century. Deaths due to diarrhea-like illnesses, including typhoid, cholera, and dysentery, represented the third largest cause of death in the nation at that time. These types of diarrheal deaths dropped dramatically once scientists identified the bacteria responsible, elucidated how these bacteria were transmitted to and among humans in contaminated water supplies, and developed effective water treatment methods to remove pathogens from water supplies.

In addition to being of food- or waterborne origin, infectious disease can be airborne, arthropod-borne (spread by mosquitoes, ticks, fleas, etc.), or zoonotic (spread by rodents, dogs, cats, and other animals). Legionellosis can be contracted from naturally occurring bacteria found in water and spread through poorly maintained artificial water systems (e.g., air conditioning, ventilation systems). Arthropod-borne diseases, including Lyme disease, Rocky Mountain spotted fever, and West Nile virus, can be contracted from certain ticks and mosquitoes that acquire bacteria or viruses by biting infected mammals or birds.

Birth defects are structural or functional anomalies that present at birth or in early childhood. Birth defects cause physical or mental disability and can be fatal. They affect approximately one out of 33 babies born each year in the United States and remain the leading cause of infant mortality (Infant Mortality indicator). People with birth defects may experience serious, adverse effects on health, development, and functional ability.25 Birth defects have been linked with a variety of possible risk factors that can affect normal growth and development—genetic or chromosomal aberrations, as well as environmental factors such as exposure to chemicals; exposure to viruses and bacteria; and use of cigarettes, drugs, or alcohol by the mother. The causes of most birth defects are unknown, but research continues to show the possible influence of environmental exposures (e.g., prenatal exposure to high levels of contaminants such as mercury or PCBs). The relationship between exposure to lower concentrations of environmental contaminants and birth defects, however, is less clear.

Low birthweight and preterm delivery are considered important risk factors for infant mortality and birth defects. Low birthweight and preterm infants have a significantly increased risk of infant death, and those who survive are more likely to experience long-term developmental disabilities.26,27 Multiple birth babies have a low birthweight rate of more than 50 percent, compared to approximately 6 percent among singletons.28 To eliminate the effect that multiple births may have on birth outcomes, this report presents data for singleton births only.

Environmental exposures are being investigated for possible associations with birth outcomes such as low birthweight, preterm delivery, and infant mortality. Some of the risk factors for low birthweight infants born at term include maternal smoking, weight at conception, and nutrition and weight gain during pregnancy.29 Specific examples of known or suspected environmental contaminant influences on birth outcomes include environmental tobacco smoke, lead, and air pollution. Environmental tobacco smoke is associated with increased risk of low birthweight, preterm delivery, and sudden infant death syndrome.30 Several studies have identified lead exposure as a risk factor for preterm delivery.31 Growing evidence shows exposure-response relationships between maternal exposures to air pollutants (e.g., sulfur dioxide and particulates) and low birthweight and preterm delivery.32,33,34 Research continues, however, in establishing causal relationships between air pollution and low birthweight and preterm birth. Researchers also continue to examine possible associations between other contaminants as birth outcome risk factors, such as pesticides, polycyclic aromatic hydrocarbons, and others.

EPA selected indicators for human diseases and conditions with well-established associations with exposures to environmental contaminants, recognizing that in most cases risk factors are multi-factorial and that the development of a particular disease or condition depends on the magnitude, duration, and timing of the exposure. The diseases and conditions addressed in this ROE question may be associated with, but cannot be tied directly to the contaminant levels or other environmental conditions reported by national-level ROE indicators in Air, Water, and Land. Learn more »

There are other diseases or conditions of potential interest for which no national-scale data are currently available, or for which the strength of associations with environmental contaminants is still being evaluated. Additional data are needed to prompt or enable EPA to track other diseases and conditions with potential environmental risk factors (direct or indirect), particularly those for which unexplained increases are being noted. Examples of diseases or conditions with suggestive or growing evidence that environmental contaminants may be a risk factor include behavioral and neurodevelopmental disorders in children, neurodegenerative disorders, diabetes, reproductive disorders, and renal disease.
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